The onset and timing of Caledonian sinistral shear in County Down

Abstract

In the south-westerly Irish continuation of the Southern Uplands ample evidence of sinistral transpression includes: (a) the clockwise transection of F 1 folds by S 1 cleavage combined with sub-horizontal extension in the cleavage plane, (b) local development of vein arrays deformed by sinistral simple shear and (c) an abundance of sinistral wrench faults. The sinistral movements are dated relative to the local history of Caledonian deformation and the intrusion of two suites of Caledonian lamprophyre dykes. Clockwise transection of folds by cleavage is not a feature of the Northern Belt or of the northern tracts of the Central Belt in County Down. Its geographic distribution suggests that sinistral transpression was first effective during the accretion of Llandovery-age tracts mid-way south in the Central Belt. Sinistral movements continued episodically during the deformation of early pre- and post-S 1 vein sets and was finally expressed in widespread wrench faulting which continued beyond the date of intrusion of the younger dykes, at about 400 Ma.